Capping of sensitive micromechanical sensor elements is typically performed by bonding/gluing a cap wafer to the completely processed sensor wafer. In order to include a defined atmosphere or a defined pressure, this process step must be carried out under the same conditions. The cap wafer is pre-structured, usually by KOH etching, to ensure the movability of the sensor structures. One of the problems with this complicated process is that when the capping is applied the sensor structures are freely movable, and therefore are extremely sensitive to impact and particle contamination.
One conventional capping, thin-layer capping technology, omits a cap wafer, and instead provides a cavity between the micromechanical structures to be exposed and a silicon layer (cap layer), which is produced using a customary deposition process. This method is based on the removal of a filling layer, situated beneath an existing silicon (Si) cap layer, by etching to produce the cavity. To feed the etching gas into the filling layer, a perforation is usually provided in the cap layer which is later resealed, after the filling. layer and optionally other sacrificial layers have been etched away, by depositing a sealing layer. However, introduction of etching gas through the Si cap layer requires fairly complicated measures to prevent the etching gas from also attacking the Si cap layer.
In this respect, a thin-layer capping (fill poly) technology is described in German Patent Application No. DE 100 06 035 A1, in which the Si cap layer is provided with a perforation composed of trenches which are produced using a conventional deep-etching process. To allow the fill epipoly Si layer to be etched by the etching gas ClF3 in a selective manner with respect to the Si cap layer, side wall passivation of the trenches, as well as further protective layers, are provided.
German Patent Application No. DE 10 2004 036 803 A1 discloses an etching method which may be used for thin-layer capping, in which a very high desired selectivity of approximately 4000:1 with respect to polysilicon may be achieved using ClF3 as the etchant in combination with a (filling) layer composed of a silicon-germanium (Si1-xGex) alloy which is to be removed. In conjunction with the production of a poly-Si cap layer, this selectivity is sufficient to channel the etching gas, without attacking the cap, through trenches conventionally produced in the cap layer using a masking technique. However, as mentioned in the cited German patent application, sealing of these relatively large trenches or openings in the cap (in the micrometer range) requires a sealing layer having a relatively large thickness of approximately 1 μm to 20 μm. For such thick sealing layers, however, there is always the risk that the exposed micromechanical structures beneath the large openings may be inadvertently coated as well. On the other hand, there is a concern that for porosified cap silicon, having openings in the nanometer range, the selectivity available in the known method is not high enough to prevent the etching gas from attacking the porous Si material, which is present in the form of small grains.
A method is described in German Patent Application No. DE 199 61 578 A1 in which the Si cap is provided with micropores. An oxide based on silicon is used as a conventional filling or sacrificial layer, and is etched away using HF vapor etching, a process which has sufficient selectivity with respect to the cap silicon, but is relatively slow.